Parallel inputs from the mediodorsal thalamus to the prefrontal cortex in the rat

Abstract

There is a growing interest in determining the functional contribution of thalamic inputs to cortical functions. In the context of adaptive behaviours, identifying the precise role of the mediodorsal thalamus (MD) in particular remains difficult despite the large amount of experimental data available. A better understanding of the thalamocortical connectivity of this region may help to capture its functional role. To address this issue, this study focused exclusively on the specific connections from the MD to the prefrontal cortex (PFC) by means of direct comparisons of labelling produced by single and dual injections of retrograde tracers in the different subdivisions of the PFC in the rat. We show that at least three parallel and essentially separate thalamocortical pathways originate from the MD, as follows: projections to the dorsal (1) and the ventral (2) subdivisions of the mPFC follow a mediolateral topography at the thalamic level (i.e. medial thalamic neurons target the mPFC ventrally whereas lateral thalamic neurons project dorsally), whereas a considerable innervation to the OFC (3) includes thalamic cells projecting to both the lateral and the ventral OFC subdivisions. These observations provide new insight on the functions of the MD and suggest a specific focus on each of these pathways for future functional studies.

Thalamic projections sustain prefrontal activity during working memory maintenance

“…As recently reported in the mosue 22 , this reflects the presence of two distinct and topographically organized MD-mPFC circuits. This topographic and reciprocal MD-PFC organization is consistent with anatomical findings in primates 19 and rats 20,21 .…”

“…Our pathway-specific experiments do not distinguish between dorsal and ventral mPFC (Supplementary Fig. 2), which reciprocally connect with lateral and medial MD in a largely segregated manner 21,22 . It is therefore possible that the distinct phase-specific behavioral impairments we observe may be attributable to activity in discrete MD-mPFC circuits, a possibility that will be important to resolve in future research.…”

“…While the anatomy of MD-prefrontal circuitry is well described in primates 19 and rats 20,21 , there have been comparatively fewer anatomical studies in the mouse. We therefore first sought to extend this literature by closely examining thalamo-prefrontal connectivity in the mouse.…”

Dorsal vs. ventral differences in fast Up-state-associated oscillations in the medial prefrontal cortex of the urethane-anesthetized rat

“…Differences in the power of the fast oscillations between subregions may also reflect the extent and nature of afferent inputs to each subregion. For example, the lateral MD nucleus has extensive arborizations to both ACC and PrL, while the medial MD projects to PrL and IL (Vertes et al 2015; Kuramoto et al 2016; Alcaraz et al 2016), but we still know very little about inputs to DP.…”

“…Certainly there is evidence for strong interregional interactions in vitro (van Aerde et al 2008), but further studies using inactivation of specific subregions would be required to reveal the extent, and direction, of interregional interactions within the mPFC in vivo. Alternatively, as outlined above, the dorsal to ventral differences may reflect different inputs to mPFC subregions (Vertes et al 2015; Alcaraz et al 2016). In addition, we have found that in the DP region the peak spindle, beta, and gamma power, relative to the onset of the Up-state, occurs much later and is more variable.…”